Sodium vapor lamp having a tin oxide coating



Nov. 30, 1965 J VAN DER w ETAL 3,221,198

INVENTOR JOHANNES VAN DER WAL PIETER M- VAN ALPHEN BY K. I i u z AGENT United States Patent 7 3,221,198 SODIUM VAIZOR LAMP HAVING A TIN OXIDE COATING Johannes van ,der Wal and Pieter Martinus van Alphen, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N .Y., a corporation of Delaware Filed Sept. 26, 1962, Ser. No. 226,337 Claims priority, application Netherlands, Oct. 4, 1961,

69,925 6 Claims. (Cl. 313-25) The invention relates to a sodium vapor discharge tube having a transparent envelope, which, preferably on the side facing the tube, is coated with a layer which passes sodium light and reflects infrared radiation.

The production of sodium light requires a minimum temperature in the discharge space of about 270 C. under the optimum operational conditions.

Since this temperature largely exceeds the normal ambient temperature, it is desirable to reduce the losses of heat to reasonable limits.

The convection losses are kept low by a vacuum sheath surrounding the discharge tube and the radiation losses are kept low by the said layers.

The invention relates to layers reducing the radiation losses.

In this respect the best results have hitherto been obtained by means of thin metal layers, particularly by means of gold layers having a thickness of about 150 A. Thus the efliciency of the sodium vapor discharge tube could be raised by about 20%, as compared with a tube without infrared-reflecting layers. This increase in efficiency per se is an important technical improvement.

However, it implies a marked reduction of the electric energy which can be supplied; it amounts to only one fourth of that with the non-improved lamp. This results in that the improved lamp, despite its higher efliciency, is capable of producing only about one third of the initial quantity of light. This means a strong reduction of the quantity oflight (lumens per cm?) produced per unit tube-volume, which means that the improved lamp is comparatively expensive and bulky.

The invention has for its object to provide an improvement in this respect.

In accordance with the invention the layer consists of tin oxide (SnO The reflection power of tin oxide for the infrared wavelength of about 5,u, which is most important for sodium lamps, is lower than that of a suitable gold layer, but its transmission coefficient for the visible sodium light is so much higher that with an at least equal eificiency the power that can be supplied and hence the produced quantity of sodium light are materially higher.

Tin oxide layers, which may be doped with at least one of the elements antimony, phosphorus, indium, fluorine and so on, are known per se for reducing heat losses due to radiation.

Satisfactory results could be obtained by means of layers, the electrical resistance of which lies between 10 and 100, preferably between 40 and 70 ohms per square cm.

It is advisable to adjust the specific load of the discharge tube, expressed in watts, dissipated in the positive column, and divided by the tube surface, at 0.14 to 0.20 w./cm.

The tube is preferably proportioned so that the current density lies between 0.20 and 0.35 amp/cm. of the inner tube diameter.

The invention will be described more fully with reference to the drawing and a table.

3,221,198 Patented Nov. 30, 1965 The drawing shows diagrammatically the construction of a sodium lamp.

The U-shaped discharge tube 1 is surrounded through out its length by a glass cylinder 2, which is open at both ends. The assembly is surrounded by an exhausted outer bulb 4, provided with a lamp base 3.

The discharge tube is provided with electrodes 5 and contains, apart from the required quantity of sodium metal, a filling of neon gas with a small addition of argon.

The inner diameter of the discharge tube 1 is about 13.5 mms., and the outer diameter about 15.5 mms. and the overall length measured from electrode to electrode along the tube axis is about 800 mms. The open cylinder 2 has a length of about 400 mms. and an inner diameter of about 50 mms. The outer bulb 4 has a length of about 500 mms. and an inner diameter of about 60 mms.

Three lamps were compared with each other.

With the lamp I the cylinder 2 Wasuncoated.

With the lamp II the cylinder 2 was coated on the inner side with a gold layer of a thickness of 150 A.

With the lamp III the cylinder 2 was internally coated with a tin oxide layer 6 of a thickness of about 1000 A. and a resistance of 50 ohms/cmP.

The following results were measured:

It appears therefrom that the new lamp III produces about 83% more sodium light than the lamp II with the gold layer of optimum size, whereas the efliciency of the lamp III is even slightly higher (133 lm./w.).

The open cylinder 2 is shown only by way of comparison with the lamp II. Both with the lamp II and with the lamp III it may be dispensed with and the infrared reflecting layer may be applied directly to the inner side of the envelope 4, of which the diameter may then be smaller.

What is claimed is:

1. A sodium vapor lamp comprising a light-transmissible envelope surrounding a sodium vapor discharge tube, said light-transmissible envelope having on the inner surface thereof facing the sodium vapor discharge tube a coating of tin oxide.

2. A sodium vapor lamp comprising a sodium vapor discharge tube and a light-transmissible envelope surrounding the discharge tube and having on the inner surface thereof facing the discharge tube a coating of tin oxide having a resistance of about 10 to ohms/cmfi.

3. A sodium vapor lamp as claimed in claim 2 in which the resistance is between about 40 to 70 ohms/cmF.

4. A sodium vapor lamp comprising a sodium vapor discharge tube operating with a positive column discharge in which the power dissipated in the positive column divided by the surface of the tube is between about 0.14 and 0.20 w./cm. and a light-transmissible envelope surrounding the discharge tube and having on the inner surface thereof facing the discharge tube a coating of tin oxide.

5. A sodium vapor lamp comprising a sodium vapor discharge tube operating with a positive column discharge and with a current density of about 0.20 to 0.35

a amp/cm. determined on the basis of the diameter of the tube, and a light-transmissible envelope surrounding the discharge tube and having on the inner surface thereof facing the discharge tube a coating of tin oxide. 6. A sodium vapor lamp comprising a first envelope containing sodium vapor admixed with an ionizable gas, a pair of electrodes within the envelope for producing a discharge, a light-transmissible envelope surrounding and spaced from said first envelope, and a coating of tin References Cited by the Examiner UNITED STATES PATENTS 6/1957 Reinker 3131l2 8/1959 Verwey 313221 X GEORGE N. WESTBY, Primary Examiner.

oxide on the inner surface of said second envelope for 10 DAVID J. GALVIN, Examiner. 

1. A SODIUM VAPOR LAMP COMPRISING A LIGHT-TRANSMISSIBLE ENVELOPE SURROUNDING A SODIUM VAPOR DISCHARGE TUBE, SAID LIGHT-TRANSMISSIBLE ENVELOPE HAVING ON THE INNER SURFACE THEREOF FACING THE SODIUM VAPOR DISCHARGE TUBE OF COATING OF TIN OXIDE. 